Can you wear an ACU/IHWCU top with combat trousers (the pants with knee pad inserts) in a field environment?

I think the SGT is asking if one can wear a top other than the combat t-shirt (baseball shirt) with the combat trousers (knee pad pants). A super quick search on AR 670-1 yielded no results so its probably going to be a local policy for your 1SG or CSM to decide.
My RQ-7B Shadow maintainers were tearing though the knees in their pants performing maintenance in the field. The work area is usually gravel. The issued army knee pads are lacking, too rounded for longer use especially when on both knees, and they don't stay in place unless they are so tight they border on painful. We managed to get some of the mechanics the combat pants to both reduce fatigue and wear and tear on uniforms.
If there is no policy perhaps help your leadership develop one and back it with a reason.

In the real world, what you wear "in a field environment" is determined by your chain of command.

Keep in mind that with any combination that isn't explicitly covered by regulations ANY level of the chain of command might find it objectionable. As a Combat Engineer you are likely to have more exposure to officers outside your immediate unit -- this could make something like this iffy. I wonder what the CG or division CSM would think of that combination...

When you are conducting engineer support for other units, you will be seen by lots of people outside your immediate chain. Some superiors are reluctant to 'interfere' with soldiers outside their direct chain, especially if they aren't sure what is or isn't allowed. On the other hand, some will make a 'correction' even when they are totally ignorant. I don't know the current climate in your unit, but I strongly suspect you would encounter a mix of both attitudes.

SGT (Join to see) Brother you'd have to ask your CoC about that. Only place I've seen them worn was while deployed.